Electrodynamic microphone having an adjustable tube for changing the directional characteristic of the microphone



Sept. 8, 1970 A. H. VAN LIEMPD 3,527,902

ELECTRODYNAMIC MICROPHONE HAVING AN ADJUSTABLE TUBE FOR CHANGING TEE DIRECTIONAL CHARACTERISTIC OF THE MICROPHONE Filed- March 21. 1967 lllll F'IG.1

F'I G,3

INVENTOR.

ADRIANUS H. VAN LIEMPD BY M AGEN United States Patent 3,527,902 ELECTRODYNAMIC MICROPHONE HAVING AN ADJUSTABLE TUBE FOR CHANGING THE DIRECTIONAL CHARACTERISTIC OF THE MICROPHONE Adrianus Hendrijus vanLiempd, Emmasingel, Eindhoven, Netherlands, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Mar. 21, 1967, Ser. No. 624,942 Claims priority, applicatiolafietherlands, Mar. 26, 1966, 66 14 Int. Cl. H04r 1/38 US. Cl. 179-121 11 Claims ABSTRACT OF THE DISCLOSURE An electrodynamic microphone in which the back side of the diaphragm communicates with a closed air chamber formed by an adjustable sleeve within the microphone housing. The backside of the diaphragm communicates either with the open air or with the closed air chamber, depending upon the position of the displaceable sleeve. Adjustment of the sleeve provides a change in the directional characteristic of the microphone with little or no change in frequency response.

The present invention relates to an electrodynamic microphone having a housing consisting of a cap permeable to sound and a shaft, while in the cap there is mounted a microphone capsule having a diaphragm which communicates on the front side with the ambience and on the rear side with a closed air chamber located at least in part in a sleeve provided in the shaft.

Such a microphone is known from the British patent specification 755,143. Due to the closed air chamber in the shaft, this microphone has a comparatively flat frequency characteristic, while the directional characteristic is cardioidal.

The invention is characterized in that the sleeve is provided in the shaft so as to be displaceable and is operated by a mechanical switch so that the rear side of the diaphragm communicates with the ambience. Thus, the advantage is obtained that with a substantially unchanged frequency characteristic, the directional characteristic can be considerably varied in a simple manner. When the rear side of diaphragm of the microphone is completely closed from the ambience, the directional characteristic of the microphone exhibits a substantially uniform course, that is to say that this so-called omnidirectional characteristic in which the sensitivity of the microphone is substantially equal in all directions. When the rear side of the diaphragm is not closed from the ambience, however, the sensitivity of the microphone on one side is an optimum, whereas it is O on the opposite side.

One embodiment according to the invention includes a sleeve enclosing the air chamber and is characterized in that the sleeve is closed at one end and is open at the other end and can be displaced inside the shaft in a longitudinal direction, while in one position of the sleeve the open end engages the rear side of the microphone capsule.

By a simple manipulation, that is to say, by a displacement of the sleeve, any directional characteristic can be obtained between the so-called omnidirectional characteristic and the cardioidal characteristic. A modification of this embodiment provides a microphone in which the sleeve tightly fits into the shaft and the mechanical switch consists of a cam provided on the outer periphery of the sleeve and located in a longitudinal slot in the wall of the shaft. In this embodiment, a helical spring presses against the closed end of the sleeve.

In a particularly favourable embodiment of the invention, the mechanical switch further includes a ring which surrounds the shaft and is provided on its inner side with an inclined guide engaged by the cam.

When the ring is rotated, the cam slides along the inclined guide into the ring; a force is then applied to the cam which causes the sleeve to move inside the shaft.

A quite different embodiment of the invention is characterized in that the air chamber is limited on one side by the shaft and on the other side by a rotatable sleeve which is arranged in the shaft with tight fit. The end of the sleeve facing the capsule is provided with one or more eccentrically arranged apertures corresponding in one position with apertures of the rear side of the capsule which communicate with the rear side of the diaphragm, one or more apertures being provided on the outer periphery both in the sleeve and in the shaft which correspond with each other in the said position. Instead of being displaced inside the shaft, the sleeve is now rotated. The directional characteristic of the microphone can be improved by providing the apertures in the sleeve embodiment with an acoustic resistance.

In a further particularly advantageous embodiment the mechanical switch consists of a cam provided on the outer periphery of the sleeve and located in a transverse slot in the wall of the shaft.

The invention will now be described more fully with reference to the accompanying drawing.

- In the drawing:

FIG. 1 is a longitudinal sectional view of a microphone in accordance with the invention,

FIG. 2 is a perspective view of a detail thereof; and

FIG. 3 is a partial longitudinal sectional view of another embodiment of a microphone in accordance with the invention.

FIG. 1 is a longitudinal sectional view of an electrodynamic microphone in which the housing 1 consists of a cap 2, permeable to and a closed metal shaft 3. Inside the cap 2 there is arranged a microphone capsule 4, the diaphragm 5 of which communicates with the ambience through a member 6 for protecting the diaphragm and with the rear side 7 through ducts (not shown). A sleeve 8 is located in the shaft 3 and is pushed at one end, by means of a helical spring 9, against a rubber ring 10 on the rear side 7 of the microphone capsule 4. The shaft 3 is surrounded by a rotatable ring 11 which is provided, on the inner side, with a plastic sleeve 12. This sleeve has a recess 13 into which fits a cam 14 on the inner side of the ring 11. The sleeve is further provided with an inclined guide 15a on which a pin 15b bears. This cam is disposed freely in a longitudinal slot 16 and is screwed to the sleeve 8.

When the ring 11 is rotated, the pin 15b is forced to move forwards 0r backwards through-the guide 15a. In the latter case, the sleeve 8 gets free from the capsule 4 and the sound can reach the rear side of the diaphragm 5 in the cap 2 through the ducts (not shown) in the capsule 4. The directional characteristic is then cardioidal. If the sleeve 8 fully engages the ring 10, the sound cannot reach the rear side of the diaphragm. The directional characteristic is then omnidirectional.

FIG. 3 shows another embodiment. In this figure, the shaft 3 is partly broken away. The shaft 3 encloses, with a tight fit, a short rotatable sleeve 8 on which the cam member 15 is provided. The cam member is adapted to move freely in a transverse slot in the shaft.

The end of shaft 3 is provided with apertures 17 which cooperate with apertures 18 in the sleeve 8. The apertures 18 are covered with gauze 19.

The sleeve is further provided with other apertures 20 which in turn cooperate with apertures -21 in the shaft 3.

The apertures 17 and 18 correspond with the apertures 20 and 21 in one position of the sleeve 8 and the shaft 3. The sound can then reach the rear side of the diaphragm. The directional characteristic of the microphone is cardioidal in this position.

What is claimed is:

1. An electrodynamic microphone comprising a cap permeable to sound and a shaft extending therefrom to form therewith a substantially enclosed housing, a sleeve positioned within said shaft so as to be displaceable in the shaft, a microphone capsule positioned within said cap and having a diaphragm which communicates on the front side with the open air and on the rear side with a closed air chamber composed, at least in part, of said sleeve, and a mechanical switch coupled to the sleeve to movably adjust the sleeve relative to the microphone capsule to provide an opening in the air chamber whereby the rear side of the diaphragm communicates with the open air.

2. An electrodynamic microphone as claimed in claim 1 wherein the air chamber is completely enclosed by the sleeve, one end of the sleeve being closed and the other end open, the sleeve being displaceable inside the shaft in a longitudinal direction so that in one position of the sleeve the open end engages the rear side of the microphone capsule.

3. An electrodynamic microphone as claimed in claim 2 wherein the mechanical switch comprises a cam member provided on the outer periphery of the sleeve and located in a longitudinal slot in the wall of the shaft, and a helical spring arranged to press on the closed end of the sleeve.

4. An electrodynamic microphone as claimed in claim 3 wherein the mechanical switch further includes a ring which surrounds the shaft and provided on the inner side with an inclined guide engaged by the cam member.

5. An electrodynamic microphone as claimed in claim 1 wherein the sleeve is rotatable and is arranged within the shaft to provide a tight fit and the air chamber is limited at one end by the shaft and at the other end by said rotatable sleeve, the end of the sleeve facing the capsule being provided with one or more eccentrically arranged apertures which in one position of the sleeve are aligned with apertures in the rear side of the capsule, said capsule apertures being arranged to communicate with the rear side of the diaphragm, and one or more apertures being provided on the outer periphery of the sleeve and the shaft so as to be in alignment with each other in said one position of the sleeve.

6. An electrodynamic microphone as claimed in claim 5 wherein one or more of the apertures in the sleeve are provided with acoustic resistances.

7. An electrodynamic microphone as claimed in claim 5 wherein the mechanical switch comprises a cam member provided on the outer periphery of the sleeve and located in a traverse slot in the wall of the shaft.

8. An electrodynamic microphone comprising a housing including a cap permeable to sound and an elongated tube extending therefrom, a microphone capsule mounted within said cap and having a diaphragm exposed to the open air on the front side, a sleeve coaxially positioned within said tube and movable in the tube between first and second positions, means including said sleeve disposed adjacent the rear side of the capsule to form a closed air chamber that communicates with the back side of the diaphragm in the second position of the sleeve, and means coupled to the sleeve for moving said sleeve between said first and second positions whereby the back side of the diaphragm communicates with the open air by providing an opening in the air chamber and with said closed air chamber, respectively. a

9. A microphone as claimed in claim 8 wherein the far end of the sleeve is closed and the end adjacent the capsule contains one or more openings, said sleeve being axially movable in the tube so that the adjacent end engages the rear side of the capsule in said second position of the sleeve.

10. A microphone as claimed in claim 8 wherein said tube is closed at one end to define an air chamber and the rear side of the capsule contains one or more apertures which communicate with the back side of the capsule diaphragm, said sleeve being rotatable about its longitudinal axis, the end of the sleeve adjacent said capsule containing one or more apertures that are aligned with the apertures in said capsule in one position of the sleeve, said sleeve and tube each including apertures in a side wall that are aligned with each other in said one position of the tube.

11. A microphone as claimed in claim 8 wherein said sleeve moving means comprises, a ring surrounding said tube and rotatable about the tube axis, and cam means coupling said ring to said sleeve so that rotation of the ring causes an axial displacement of the sleeve within said tube.

References Cited UNITED STATES PATENTS 3,141,070 7/1964 Schenkel 179l2l RALPH D. BLAKESLEE, Primary Examiner US. Cl. X.R. 179115.5 

